Musnad Abu Ya'la al-Mawsili 42:270

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Sa'id bin Abi 'Aruba informed us, from Qatada, from Sa'id bin al-Musayyib, from Abu Sa'id al-Khudri: That a servant of the Prophet, may Allah bless him and grant him peace, brought fresh dates. The dates of the Messenger of Allah, may Allah bless him and grant him peace, were *ba'l* (rain-fed) dates that had dryness in them. The Messenger of Allah, may Allah bless him and grant him peace, said to him, "Where did you get this date from?" He said, "This is a *sa'* that I purchased for two *sa's* of our dates." The Messenger of Allah, may Allah bless him and grant him peace, said, "This is not permissible. But if you want that, then sell your dates, then buy whichever dates you wish."